I know of a plugin for Blender called Suicidator's City Generator. Even with the free version, it's quite nice. I think it doesn't have textures unless you buy it though. Blender exports in a ton of formats, so you can use that for just about anything.
I Think I know of a City Model that would work from the Source engine. I just don't know to what degree of "Big" you mean. also, Can we expect a scaley model any time soon?
http://cgchan.com/suicidator/
https://www.youtube.com/watch?v=HROU3gLcgfg
https://www.youtube.com/watch?v=XCEN0qQOsIA
And sorry, no reptilians in the works!